In recent years, in the research field of cell biology, molecular biology, and the like, the necessity to observe biological cells such that a green fluorescent protein (GFP; Green Fluorescent Protein) or a luciferase gene which is a bioluminescent enzyme is made to function as a reporter of expression, and a fluorescent sign or a luminescent sign is attached to a specific portion or a functional protein in a cell, has grown.
In observation using a GFP, because the GFP is protein generating fluorescence in accordance with irradiation of an excitation light, and fluorescence is obtained by irradiating a high-intensity excitation light onto a specimen on which the GFP has been acted, the specimen is easily damaged, and an observation time is limited to about one or two hours. However, in observation using a luciferase, because the luciferase is a self-luminous enzyme, and an excitation light causing damage to a specimen is not required, observation for about five days is possible. On the other hand, in observation using a GFP, for example, it is possible to increase a quantity of generated fluorescence by converging an excitation light onto one point of a specimen by a confocal laser scanning microscope. However, in observation using a luciferase, because a light quantity cannot be increased by an excitation light, it is necessary to observe a specimen with low light from the luciferase.
Generally, there are a broad range of applications to detect low light, and not only in observation using a luciferase, but also in observation using a GFP, there are a case in which observation is carried out by lowering an excitation light, photometry of fluorescence from a DNA chip, dark visual field observation of flagellum of micro organism, and the like. A high-sensitivity cooled CCD camera has been actively developed for such applications.
Further, in an application to detect low light, in order to be able to condense more light from a specimen, conventionally, an objective lens having large NA (numerical aperture) is used as an optical system which forms an image of a specimen. Note that there is a case in which an NA at an imaging side is made greater by disposing a demagnifying lens at an image side of an imaging lens of a microscope separately from a case in which an NA at a specimen side is made greater by an objective lens. However, an object thereof is, not for observing low light, but to conform sizes of a visual field of a visual observation and a visual field imaged by a CCD.
FIG. 28 shows one example of an imaging optical system in which a demagnifying lens is disposed. As shown in FIG. 28, a demagnifying lens 104 is disposed in a space between the imaging lens 103 and an image surface 106 which is an image space of the imaging optical system formed from an objective lens 102 and an imaging lens 103, which forms an imaging optical system telecentric as a whole at the image side. An object point 101a on an optical axis OA3 and an object point 101b out of the optical axis OA3 on a specimen 101 are respectively imaged onto image points 106a and 106b on the image surface 106 when the lens 104 is not disposed. However, when the lens 104 is disposed, those are respectively imaged onto image points 105a and 105b. Further, in this example, the image point 105b is imaged at a height about half that of the image point 106b. Note that a position and an aperture of exit pupil Pu of the imaging optical system are set to be not changed regardless of the presence or absence of the disposition of the lens 104.
Further, the imaging optical system telecentric to an image side as shown in FIG. 28 is conventionally commonly used for a length measuring microscope or the like, and is used as an optical system dispensable for a CCD camera in recent years. The imaging optical system telecentric on an image side is an optical system whose exit pupil is positioned at infinity, and is an optical system in which chief rays which are ejected from the optical system and go toward respective image points are made parallel to the optical axis. Usually, because the sensitivity of a CCD camera is reduced as an incident angle of light with respect to an imaging area is made greater, in order to take an image evenly and at high sensitivity on the entire imaging area, it is necessary to make chief rays of light incident into the respective pixels of the CCD camera perpendicular to the imaging area, and in order to realize this, an imaging optical system telecentric on an image side is essential.
In addition thereto, the imaging optical system telecentric on an image side is commonly used for a microscope or the like (for example, refer to Patent Documents 1 and 2). In a microscope disclosed in Patent Document 1, it is possible to maintain the image side to be telecentric even when the objective lens is replaced by disposing a replaceable optical unit in accordance with a change in an exit pupil position associated with the replacement of objective lenses. Further, in a microscope disclosed in Patent Document 2, it is possible to obtain a sharp observed image without generating an interference pattern on an imaging area even when a laser source is used by slightly inclining an imaging area of the CCD camera with respect to the optical axis.
Note that, the development for making a high-resolution CCD camera has been carried out separately from making a high-sensitivity CCD camera, and a high-definition CCD camera in which pixel sizes are 2 to 3 μm, and a pixel count is five millions has been realized. Then, an apparatus called a virtual slide has been developed by combining such a high-definition CCD camera and a microscope. In a virtual slide, a plurality of images taken after dividing a specimen on a preparation into a plurality of areas are acquired in advance by using an imaging optical system of about 20 magnifications, and in which an image surface curvature and distortion are suppressed to be less, and after the acquired respective images are joined together on image data, an image of arbitrary magnifications of about 5 to 100 magnifications is displayed on a monitor by electronic zooming serving as electronic enlargement processing. In this way, because it is possible to display a high-definition image of a specimen even without any actual microscope and specimen, a virtual slide is utilized as an educational tool for medical students.
Patent Document 1: Japanese Patent No. 2990871
Patent Document 2: Japanese Patent Application Laid-Open No. 2000-235150
Patent Document 3: Japanese Patent Application Laid-Open No. 2004-191252
Nonpatent Literature 1: David K. Welsh, Seung-Hee Yoo, Andrew C. Liu, Joseph S. Takahashi, and Steve A. Kay, “Bioluminescence imaging of Individual Fibroblasts Reveals Persistent, Independently Phased Circadian Rhythms of Clock Gene Expression”, Current Biology, 2004, Vol. 14, p. 2289B295
Nonpatent Literature 1: N. Takasuka, M. R. H. White, C. D. Wood, W. R. Robertson, and J. R. E. Davis, “Dynamic Changes in Prolactin Promoter Activation in Individual Living Lactotrophic Cells”, Endocrinology, 1988, Vol. 139 p. 1361-1368